1. Field of the Invention
The present invention relates to a linear motor which employs a piezoelectric element, and an optical apparatus in which a lens is moved by such linear motor.
2. Related Background Art
In recent years, progress in electronic technologies has increased the demand for developing fine motion driving sources or high-accuracy fine motion mechanisms in the field of mechanics. Accordingly, driving sources or fine motion mechanisms which utilize fine deformity of an object or of a substance have been used as the driving source for the fine moving of mechanisms or for finely positioning mechanisms. The most commonly used driving source among them uses a piezoelectric element, and various types of such piezoelectric type motors have been proposed, some of which have been put into practical use. However, most of the previously proposed piezoelectric motors have not yet been put into practical use because they still have drawback to be solved.
The fine and linear motion mechanism disclosed in Japanese Patent Laid-Open No. 55-100059 is an example of an application of a piezoelectric linear motor constructed using a piezoelectric element, and has a potential of being put into practical use.
FIG. 11 schematically shows a linear fine motion mechanism or a piezoelectric linear motor disclosed in the aforementioned patent application. The piezoelectric linear motor has laminated piezoelectric elements 101, 102a, 102b, 103a and 103b. These piezoelectric elements are held by a pair of retaining members 104 and 105 each having a T-shaped cross-section, as shown in FIG. 11. The first laminated piezoelectric element 101 can expand and contract parallel to a guide rail 106 in a direction indicated by A. The second laminated piezoelectric elements 102a and 102b and the third laminated piezoelectric elements 103a and 103b can expand and contract in a direction perpendicular to the direction indicated by A, i.e., toward the opposing wall surfaces of the guide rails 106.
This piezoelectric linear motor can move in the direction indicated by A or in a reverse direction in a worm-like fashion by sequentially changing the timing in which a voltage is applied to the first to third laminated piezoelectric elements 101, 102a, 102b, 103a and 103b according to a predetermined pattern. Hence, the distance through which the piezoelectric linear motor moves can be increased by increasing the length of the guide rails 106.
The aforementioned conventional piezoelectric linear motor employs laminated type piezoelectric elements. However, the laminated type piezoelectric elements have a low mechanical accuracy for reasons involving the structure and manufacturing method thereof, and thus cannot provide a stable operation accuracy. More specifically, since the laminated type piezoelectric element is manufactured by laying tens or hundreds of piezoelectric plates on top of one another and then bonding them using an adhesive, the thickness thereof varies. Moreover, the displacement of the laminated type piezoelectric element is not large. For these reasons, the pressing force of the laminated type piezoelectric element against the guide rail varies, reducing positional accuracy or causing operational disability. Furthermore, the use of the laminated type piezoelectric elements increases the thickness of the motor, which makes it difficult to reduce the size of the motor.
In imaging devices, such as video cameras, which employ a rear focus type zoom lens, an electromagnetic motor, such as a step motor or a voice coil motor, is used as the focusing motor (U.S. Pat. No. 4,920,369). When the step motor is used as the focusing motor, it is not necessary to provide a lens position detection device, because lens position can be detected by counting the number of driving pulses. However, the step motor is large in size and the use thereof makes reduction of a lens unit impossible. The use of other types of electromagnetic motor makes reduction in the size of the motor possible. However, it requires provision of a lens position detection device, and hence makes reduction in size of the motor impossible. Reduction in the size of an imaging element (CCD) has increased the demand for a fine lens positioning operation. However, a combination of an electromagnetic motor, such as a step motor, and a transmission mechanism, such as a feed screw mechanism, cannot achieve the required accurate lens positioning.